This invention relates to fuel cell systems, and in particular, to apparatus and method for operatingg such systems.
In fuel cell systems, the tendency has been to operate the system fuel cell at higher process gas pressures to obtain greater fuel cell efficiency. This is discussed, for example, in the following U.S. Pat. Nos. 3,972,731; 3,973,993; 3,976,506; and 3,976,507. Increasing the process gas pressure, however, has certain disadvantages when fuel cell systems are operated as heretofore contemplated. Thus, in present day systems oxidant process gas and fuel process gas pressures are maintained substantially the same (e.g., a pressure differential of less than about 1 psi is customary). Increasing the operating pressure of the fuel cell is thus generally carried out by increasing both process gas pressures (see, e.g., the aforementioned U.S. patents). Since the fuel gas pressure is determined by the pressure of steam used in the system fuel processing equipment, i.e., the system fuel gas reformer, increased fuel process gas pressure necessitates increased steam pressures. Increasing the steam pressure required for fuel processing, i.e., for reforming reaction in turn, requires higher cell operating temperatures, since the fuel cell waste heat is generally utilized for steam production. Increased fuel cell temperatures; however, result in shorter fuel cell life.
U.S. application Ser. No. 226,801 filed Jan. 21, 1981, assigned to the same assignee hereof, discloses a system which enables the production of high pressure steam without increased fuel cell temperature through the use of an auxilliary heat exchanger which responds to the exhaust oxidant gas and to a higher temperature gas derived elsewhere in the system and which through heat exchange increases the exhaust oxidant gas temperature. The oxidant gas at the higher temperature is then applied to a steam generator for generating steam at the pressure necessary to produce fuel process gas at the required high pressure (e.g., at a pressure in the range of about 30 to 200 psia).
While the above disclosed system contemplates one method of enabling high fuel cell pressures other techniques are still being sought.
It is an object of the present invention to provide an improved fuel cell system capable of high pressure operation.
It is a further object of the present invention to provide a high pressure fuel cell system having increased life and efficiency.